class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        i = 0
        while i < len(nums):
            if i == len(nums) - 1:
                return "No solution here!"
            r = target - nums[i]
            # cannot use a num twice
            num_follow = nums[i + 1:]
            if r in num_follow:
                return [i, num_follow.index(r) + i + 1]
            i += 1